Nicola Tesla's alternating current (“AC”) electricity powers appliances and machinery connected to the electric grid because 100 years ago, Tesla proved that his transformers could easily change AC voltage. Electrical equipment, however, increasingly is designed for (or already was compatible with) DC power but is forced to accommodate AC power. For example, computers, cell phones, printers, televisions, modern heat pumps and air conditioners use direct current and have to convert AC to DC power before they can use the electricity. Furthermore, the most inexpensive form of electricity going forward is direct current from solar electricity. Thus, AC and DC must be interconverted twice, with attendant losses and the need for equipment costs to do so.
Having to convert DC to AC and then back to DC again increasingly limits efficiency, in part also because of voltage and phase changes needed. Increasingly, modern appliances such as computer and cell phone power adapters, induction stoves and heat pumps need DC and unnecessarily have to deal with AC power issues such as power factor correction.
Inefficiency also arises from induction loads having big turn-on surge currents such as vacuum pumps used in refrigerators. The instantaneous turn on current of such load may be many times higher than the running load. This causes high equipment costs because the inductor used by a DC to AC inverter to change voltage will saturate at high current pulses and limits power throughput based on that starting pulse. This forces the use of an expensive and inefficient large inverter for a small appliance that may need 160 watts to run, but an 800 watt inverter in order to start. A distribution system for DC power that avoids one or more inductor based voltage conversions would provide tremendous cost and efficiency advantages. Accordingly, any technology that can allow more direct use of DC power from the premier future power source (solar electric) by the end user will lower running costs, infrastructure costs and control costs.
An underappreciated problem of using DC power for appliances is that the on-off switches for these appliances are designed for utility supplied 60 hertz alternating current. DC current is more damaging to switches because when a switch opens up to disconnect, a long spark is formed. AC power goes to zero volts at 120 times per second, which allows a spark to break apart more easily, than a spark formed by continuous direct current. A variety of techniques have been explored to alleviate the spark problem. See for example WO2015021010A1 “Commutating switch with blocking semiconductor” by Faulkner et al. and U.S. Pat. No. 9,450,394 by Ohlsson et al.
The limitation of switch contacts for DC power has grown with the boom in solar electric DC power, which requires switches and circuit breakers that can handle high DC current loads. DC compatible switches and circuit breakers for the same voltage and current capacities as AC circuit breakers in particular cost at least several times more. New technology is needed to lower these costs as well.